Embodiments of the present invention relate to polishing and refurbishing a silicon-on-insulator transfer wafer.
In the fabrication of semiconductor devices, substrates are processed by forming or etching material on the substrates. An example of a substrate process is a process to form a silicon-on-insulator (SOI) structure on a production substrate, the SOI structure comprising a silicon layer overlying an insulator layer, which is typically an oxide layer. In one version of a SOI formation process, an oxide layer is formed on the surface of a first silicon wafer, and the surface is pressed against and bonded to a second silicon wafer surface. The bonded wafers can be heat treated to strengthen the bond attachment. The exposed surface of one or more of the wafers is then ground away to leave a thin silicon layer overlying an oxide layer on a silicon substrate. However, this process is often undesirably inefficient, as mechanical grinding or etching of the wafer can take an unsuitably long time. The process can also be costly, as a substantial amount of silicon material is wasted in the grinding process, and a fresh silicon substrate is typically required for each new process.
In an improved SOI transfer process, a SOI structure is transferred onto a production wafer from a transfer wafer by bonding the transfer wafer and production wafer together, and then de-laminating the transfer wafer from the production wafer to form the SOI structure. The SOI transfer wafer can comprise a silicon material such as a silicon wafer or silicon layer, which is implanted with ions, and an overlying thermally grown silicon oxide layer. In one version, the transfer wafer comprises a surface layer of epitaxially grown silicon which is implanted with ions, and covered with a layer of silicon oxide. The surface of the silicon oxide layer on the transfer wafer is contacted with a surface of a production wafer, which is typically a bare silicon wafer, and the wafers are adhered to one another. A subsequent de-lamination step is performed to cleave or de-laminate the transfer wafer from the production wafer at the ion-implanted layer, leaving the oxide layer and a portion of the remaining silicon material bonded to the production wafer to provide the SOI structure.
The de-laminated transfer wafer typically has a remaining portion of the silicon material, on which an oxide layer can be re-formed to perform additional SOI transfer processes. However, because the periphery of the transfer and production wafer are typically not bonded as strongly as the central region, a circumferential lip often forms about the periphery of the transfer wafer after the delamination process. The circumferential lip comprises un-transferred portions of the silicon material, as well as residual oxide insulator material. Such a circumferential lip is undesirable because it impedes surface contact between the transfer and production wafers, and limits the ability of the transfer wafer to be re-used for subsequent SOI processes. Accordingly, it is desirable to have a method of refurbishing a transfer wafer to permit transfer of SOI layers onto a plurality of subsequent wafers.
In one version, the transfer wafer is refurbished by performing a double-sided polishing step, such as by chemically mechanically polishing the wafer, to remove remaining oxide material from both sides of the wafer and polish away the circumferential lip. In yet another version described in U.S. Pat. No. 6,284,628 to Kuwahara et al, issued on Sep. 4, 2001 and assigned to Shin-Etsu Handotal Co., Ltd., which is herein incorporated by reference in its entirety, an overlying oxide layer is removed by a chemical etching step. Afterwards, the smaller step remaining on the wafer is removed by polishing the surface of the wafer just slightly, by inserting the wafer between polishing turn tables.
However, such double-sided polishing methods can be inefficient, often fail to remove all of the circumferential lip, and can require an undesirably long polishing time, for example, a double-sided polish process may only remove about ½ a micron/minute of material. The procedures can also be expensive due to the quantity of slurry and other consumable chemical mechanical polishing pads required. Also, conventional polishing procedures may remove an undesirably large amount of the silicon layer from the transfer wafer to remove the circumferential lip, reducing the number of times the transfer substrate can be used. Conversely, if the transfer wafer is underpolished, a small circumferential bumper ring, typically having a height of around 200 nm, can remain on the wafer. The remaining bumper lip can impede contact between the transfer wafer and a production wafer, and reduce the quality of the SOI transfer process. For example, for recycled transfer wafers having a circumferential bumper with a height of about 50 nm, the SOI transfer yield may be reduced by about 20% over fresh transfer wafers having planar surfaces. Overpolishing of the circumferential lip with respect to the rest of the wafer can result in edge roll-off wafer surface topography, which also reduces SOI transfer yields.
Accordingly, it is desirable to recycle and reclaim a SOI transfer wafer to re-use the transfer substrate. It is furthermore desirable to reclaim a SOI transfer wafer to provide a transfer wafer that is substantially absent a circumferential lip, to provide a substantially planar contact surface.